In a bowl, mix the onion with all the other ingredients up to and including the tomato sauce, season.
To prepare the peppers
Slice a lid approx. 3 cm wide off the top of each pepper. Transfer the peppers to the prepared dish.
To fill the peppers
Fill the peppers with the rice mixture, spread the parmesan evenly over the top, place the lid on top.
Bake for approx. 45 mins. in the lower half of an oven preheated to 200°C.